"Unicorn", the closest black hole to Earth, discovered in space

How small can a black hole be?For several decades, astronomers have tried to answer this question by counting cosmic monsters in our corner of the universe. The past years have been rich in scientific discoveries - researchers have managed to find many black holes of different sizes, including a supermassive monster in the very heart of the Milky Way. Interestingly, until recently, astronomers have not observed any signs of the existence of small black holes. This is a long-standing mystery in astrophysics. Now scientists have discovered a black hole with a mass of only three times the mass of our Sun, making it one of the smallest black holes discovered to date. But the most remarkable, perhaps, is that the new black hole, unofficially named "Unicorn", is also the closest such object to our planet. According to the data obtained during the work, the "Unicorn" is located only 1,500 light years from Earth.

Ohio State University astronomers have discovered the closest black hole to Earth. She was named the Unicorn in part because of her diminutive size.

In search of the invisible

Since no light can escape fromblack hole, these objects can be detected only indirectly. For example, most of the known black holes were discovered by searching for X-rays emitted when an invisible object rips material from an orbiting companion star - when this material is heated in a dense ring around a black hole (accretion disk), it emits radiation that can be detected from using X-ray telescopes. The unicorn, however, was found in a different way.

Ohio State University researcherspurposefully looking for stars orbiting something invisible and massive. Their attention was ultimately attracted by the red giant star 2MASS J05215658 + 4359220 in the constellation Auriga. The study authors, published in the Monthly Notices of the Royal Astronomical Society, used data from a number of observatories to measure the periodic variations in brightness and spectrum of light emanating from the red giant, abbreviated as V723 Mon. Interestingly, these types of observations have been used for decades to find exoplanets that are extremely difficult to detect.

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Taking a close look at the red giant, scientistscame to the conclusion that he was attracted by an invisible companion object, distorting the star in the form of a raindrop. As you might guess, the companion turned out to be a "unicorn", the mass of which the researchers estimated at about 3.3 solar. It should also be noted that the data obtained gives the combined mass of both objects, and if the star is heavier than the team's estimate, it is entirely possible that the invisible object is a neutron star. But the authors of the scientific work believe that the satellite is most likely a small black hole.

This is what a black hole looks like in NASA animation.

Although the unicorn changes the shape of the red giant, itdoes not pull material from it. This means that it has no accretion disk and therefore no X-rays, so it has gone unnoticed until now. The absence of X-rays in such "quiet" black holes may explain their absence on the map of the Universe.

Interestingly, before the discovery of the unicorn, there wasseveral other candidates for small black holes have been proposed. According to National Geographics, in 2019 the same team announced that it had detected a dark object orbiting a giant star, however, the estimates of the object's mass were less accurate, and they could only conclude that it was “either a black hole or an unexpectedly massive neutron star".

Last year, another group of astronomersdiscovered what they believed to be a triple system, about 1,100 light-years from Earth, containing a black hole about four solar masses orbiting two stars. If the system did contain a black hole, it would be the closest to Earth, but other studies have questioned this discovery.

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Collapse in space-time

Astronomers hope that the unicorn and otherssuch objects will shed light on the physics that governs the formation of both black holes and neutron stars. Recall that both objects are formed when a star reaches the end of its life, depleting its reserves of nuclear fuel. But what fate awaits any single star depends on its mass.

If the star is slightly larger than our Sun, itexplodes in a supernova explosion. The rest of the star is compressed by gravity, forming a neutron star - an object so dense that material is packed together as tightly as an atomic nucleus. But if the object is much heavier, then it collapses under the influence of gravity, creating a black hole.

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Although the star may have lived ten million years, this ending is played out with incrediblespeed. "In the interval from one to five seconds, the star" decides "whether it will explode as a supernova and produce a neutron star, or it will collapse and form a black hole," - note the authors of the new study.

It is possible that the universe is inhabited by small black holes.

“Our discovery implies that in the vastThere are many small black holes in the universe that we could find if we expanded our search, ”said Tarinda Jayasinghe, an astronomer at Ohio University and lead author of the study, in a press release.

Studying the unicorn and other similar objects, inIn the future, researchers hope to get a clearer picture of what happens to stars in the last moments of their lives. Moreover, as more data becomes available, astronomers hope to know whether the lack of small black holes indicates some new aspect of stellar physics, or whether small black holes are indeed scattered throughout the galaxy, and we simply cannot count them.